mg/L
Phosphate
What is it
Phosphate (PO₄³⁻) is an inorganic compound derived from phosphorus. In an aquarium it accumulates mainly through decomposing food, especially poor-quality dry food and frozen mixes, organic waste and sometimes directly from tap water or pH buffers.
Like nitrate, it is not highly toxic to fish, but it plays a major role in the tank's biological balance, particularly in competition between plants and algae.
In planted tanks, especially high-tech tanks, phosphate is a fundamental macronutrient: the P in N-P-K, nitrogen-phosphorus-potassium. It is often dosed artificially to ensure vigorous growth.
Why it matters
Phosphate is crucial for plant energy metabolism because it forms ATP, the energy molecule, and for root development. Without sufficient phosphate, plant growth stops regardless of how much light and CO2 you provide.
Excess phosphate, often above 2.0-3.0 mg/L, has historically been considered the main cause of algae blooms, including BBA, beard algae and cyanobacteria. Modern aquarium practice shows that high phosphate itself does not cause algae; imbalance between phosphate, nitrate and CO2 does.
Limiting phosphate to zero does not remove algae. It stops plant growth and can paradoxically allow algae, which need only infinitesimal traces to survive, to take over.
Ideal ranges
Interactions with other parameters
Like nitrogen, phosphorus works in synergy with other nutrients. Phosphate must be balanced with nitrate, potassium, trace elements, light and CO2; looking at PO4 alone rarely explains algae. Severe phosphate excess (>5 ppm) in hard water can precipitate calcium and trace elements such as iron, making the water milky.
| Tank type | Min | Max | Unit |
|---|---|---|---|
| Tropical community | 0.1 | 1.0 | mg/L |
| Planted high-tech | 1.0 | 3.0 | mg/L |
| Planted low-tech | 0.2 | 1.5 | mg/L |
| Shrimp tank | 0 | 0.5 | mg/L |
Out of range: what happens
Deficiency, with PO₄ near 0, causes stunted plants, old leaves that fall prematurely, and unusually dark-green or purple leaves. Green Spot Algae (GSA) can appear explosively on glass and slow-growing leaves such as Anubias.
Moderate excess (2.0-4.0 mg/L) presents no fish risk. If other nutrients (N, K and CO2) are balanced and abundant under the Estimative Index method, plants can thrive; if they are scarce, the risk of major algae blooms rises.
Severe excess (> 5.0-10.0 mg/L) can cause mineral precipitation because phosphate binds calcium or iron, making water cloudy and nutrients unavailable. It can stress some fish and disrupt osmotic balance; in shrimp tanks, especially Caridina cantonensis, high phosphate suppresses breeding.
Common Myths
- Phosphate automatically causes brush algae (BBA); BBA is caused 99% by CO2 fluctuations, not phosphate.
- Removing all phosphate helps plants or changing only 10% of the water removes an accumulation; it can actually stop growth, and the small change removes too little to matter.
Measurement and adjustment
How to measure
Use a liquid reagent test such as JBL PO₄ or Salifert. Many inexpensive tests measure only free inorganic phosphate and are notoriously imprecise or hard to read at aquarium-relevant concentrations (0-2 mg/L).
For high-tech tanks or methodical nutrient dosing, a digital photometer such as the Hanna Checker Phosphorus HI736 or HI713 offers much greater precision and avoids interpreting nearly identical blue shades.
Testing once weekly or every two weeks is enough to calibrate dosing or water changes. Test source water too if phosphate rises despite light feeding, and test especially before and after fertiliser dosing.
How to adjust
To lower phosphate, regular substantial water changes (50% per week) are the preferred method. Siphon the substrate to remove decomposing detritus, avoid food rich in ash and phosphorus, and rinse frozen foods or feed smaller portions if phosphate rises quickly. In extreme cases, use phosphate-removing filter media such as PhosGuard, cautiously in planted tanks.
To raise it in high-tech tanks, dose monopotassium phosphate (KH₂PO₄) powder or commercial liquid fertilisers. Dosage depends on plant mass, but the usual target is 1.0-2.0 mg/L.
For empirical balance, many aquarists use a Redfield ratio of 10:1 or 15:1 by weight between nitrate (NO₃) and phosphate (PO₄). If you have 15 mg/L NO₃, aim for about 1 mg/L PO₄.
Pro Tips
- Green spot algae on slow leaves often means PO4 is too low relative to light.
- Rinse frozen foods or feed smaller portions if phosphate climbs quickly.